The present invention generally relates to radiation oncology, and more particularly to methods for oncological treatment with linear accelerators.
Radiation-emitting devices are generally known and used for radiation therapy in the treatment of patients. In a typical radiation therapy device, the gantry of a linear accelerator is swivelled around a horizontal axis of rotation in the course of a therapeutic treatment of a patient. The linear accelerator generates a high-energy radiation beam (referred to herein as a xe2x80x9cphoton beamxe2x80x9d or xe2x80x9cphotonsxe2x80x9d) for use in the therapeutic treatment.
Historically, linear accelerators used in radiation therapy applications have been equipped to provide only a single energy photon beam. In the recent past, however, some linear accelerators have been equipped to provide two different energy beams. The limited number of energies available is a continuing problem for physicians and physicists, since it is not always possible for them to give the most efficacious treatments. For example, the most commonly available dual energy linear accelerator provides six megavolt (MV) photons and either ten or fifteen MV photons. This is a typical combination since, e.g., in the treatment of breast cancer, an irradiation treatment of the whole breast is best accomplished with six MV photons. For tumors located deep within the body, however, the most commonly used energies are ten MV and fifteen MV photons. Of course, there are other tumor sites that are best treated with four MV or eight MV photons. Four MV photons are very effective for the treatment of tumors near the skin surface, while eight MV photons are excellent for the irradiation of larger breasts. Four MV and eight MV photon energies are usually not incorporated in dual energy linear accelerators because they are not needed all the time, and thus are not considered cost effective. Currently, certain manufacturers are attempting to provide linear accelerators with the capability of generating three different photon energies. Such machines, however, will still preclude many other intermediate energies that may be useful.
A therapeutic x-ray beam produced by a linear accelerator is characterized by the amount of energy that will be deposited at a treatment site by that particular x-ray beam. This characterization relates to the depth (usually measured from the surface of the skin) at which the beam""s maximum energy is deposited (often referred to in the art as xe2x80x9cdmaxxe2x80x9d). In radiation therapy, the energy deposited by ionizing radiation (absorbed dose) is typically measured in xe2x80x9cgraysxe2x80x9d or xe2x80x9cGyxe2x80x9d, instead of the traditional unit of measure of absorbed dose, the xe2x80x9crad.xe2x80x9d It will be understood that a dose of one Gy will deposit one joule of energy per kilogram of matter, and that one rad is equal to 1 one-hundredth of a Gy, or one xe2x80x9ccGy.xe2x80x9d Thus, dmax is often represented in the form of absorbed dose in units of cGy. The absorbed dose depends upon the exposure (time and intensity) as well as the inherent characteristics of the absorbing matter, i.e., density, atomic number, etc.
For example, assuming a ten by ten cm (centimeter) field, a low energy x-ray beam (e.g., six MV) would deposit the maximum energy at one and a half cm from the surface of the skin, while a high energy beam, such as fifteen MV, would deposit the maximum energy at three cm from the surface of the skin. Thus, with a fifteen MV beam, any point closer than three cm to the surface of entry would be less than dmax and any point more than three cm from the surface of entry would also be less than dmax. At locations that are nearer to the surface or further away from the surface than the location of dmax, radiation energy is deposited to a lesser degree and at a lower rate. A significant difference between higher energy beams and lower energy beams is that the higher energy beams are more penetrating. That is, the amount of radiation deposited at a given point (say five cm) beyond the location of dmax is higher for a fifteen MV beam than it is for a six MV beam. Thus, a fifteen MV beam is more effective in treating a deep seated tumor (say fifteen cm from the surface/skin) than a six MV beam.
A standard technique for treating a breast is called xe2x80x9ctangential beams.xe2x80x9d Here the radiation is given by two nearly opposing beams arranged at an angle so that they just skim the chest wall (to minimize radiation exposure to the lungs) but otherwise treat the entire breast. The base of a human breast generally provides the widest separation, (i.e., the greatest transit distance through tissue, sometimes as much as twenty-five cm. With such a wide separation, a more penetrating beam is needed. However, with a fifteen MV beam, which deposits a maximum energy at three cm, the beam may not be treating the breast tissue adequately near the skin surface. However, if a six MV beam is used, it may be more appropriate in covering the breast tissue near the skin surface, but may not be treating the deep tissue adequately.
One solution in the art has been to increase the amount of radiation that is given for each treatment so that the tissue at the center of the base of the breast receives an adequate amount of radiation with a six MV beam. Unfortunately, a consequence of this method is that the tissue near the skin becomes so xe2x80x9chotxe2x80x9d (receiving too much radiation) that the breast exhibits significant skin side effects (almost like a severe sun bum). In such a case, an eight MV beam would be more appropriate, since it would treat the breast tissue near the skin appropriately but yet more penetrating than the six MV beam. Unfortunately, eight MV beam machines are not commonly used at healthcare facilities.
Numerous methods and apparatus have been disclosed in the prior art for optimizing radiation therapies. For example, in U.S. Pat. No. 6,038,283, a method and apparatus for determining an optimized radiation beam arrangement for applying radiation to a tumor target volume while minimizing radiation of a structure volume in a patient is disclosed. The method uses an iterative cost function based on a comparison of desired partial volume data, which may be represented by cumulative dose volume histograms and proposed partial volume data for target tumors and tissue structures. This arrangement provides for the delivery of the optimized radiation beam arrangement to the patient by a conformal radiation therapy apparatus.
U.S. Pat. No.6,142,925 discloses a method and system for increasing resolution of a radiotherapy system to achieve virtual fractional monitor unit radiation delivery. The method identifies a desired treatment dose that exceeds the resolution of a radiation treatment device, and develops a schedule of treatment sessions for delivering the desired treatment dose that produces a combined treatment dose equaling the desired treatment dose without exceeding the resolution within each treatment session.
U.S. Pat. No. 5,880,477, discloses a method and apparatus for real time control of the dose rate of particles or ionizing radiation, especially X-ray radiation, generated from an electron linear gun and applied to polymer resins using an appropriate ionization chamber having planar electrodes placed in the field of particles. The method involves sampling continuously the current for collecting the load between the electrodes. The instantaneous dose rate of the radiation is represented by the collecting current. Using an appropriate shielded conducting system, the collecting current is directed to an amplification and measurement circuit arranged outside of the irradiation zone. The intensities of the current are translated into dose rate values. The does rate values are then processed and/or displayed and/or recorded.
U.S. Pat. No. 5,668,847 discloses a radiation emitting device for therapeutic radiation treatment which adjusts the actual radiation delivered to an object via a radiation beam, and which is dependent on the dimensions of an opening in a plate arrangement provided between a radiation source and an object. In this way, the radiation output has a constant wedge factor over an irradiation field, regardless of the size of the opening. The wedge factor is defined as the ratio between a reference radiation output along a reference axis of the beam with a predetermined physical wedge in the beam path and an actual radiation output of the beam in a substantially lossless beam path.
U.S. Pat. No. 5,647,663 discloses a method of radiation treatment planning for radiation systems providing multiple beams of independently adjustable intensities which limits the iterative beam weight determination to a set of discrete beam weights. This method avoids errors in post-optimization truncation of the beam weights, and decreases the iteration time. Those beams having a greatest effect on the solution are preferentially adjusted and larger changes between discrete values of beam weights are given preference to smaller changes.
U.S. Pat. No. 5,602,892, discloses a method for optimization of radiation therapy planning based on a Dynamically Penalized Likelihood (DLP) algorithm. The target function of the DLP algorithm contains likelihood terms and penalty terms connected to the likelihood terms by a number of dynamically updated penalty coefficients. The method results in a highly uniform dose to the tumor or radiosurgery volume, at the expense of some non-uniformity in the dose delivered to defined sensitive tissues.
U.S. Pat. No.5,418,827, discloses a radiation therapy apparatus for irradiation of a tumor at 360xc2x0 about the tumor within a plane. The apparatus determines a distribution of charges in a conductor that would produce a potential energy field matching the desired dose to the tumor in the plane. The fluence of any given ray through the tumor is determined by summing the charges along the ray""s path. The distribution includes areas of no irradiation which may require negative fluences. Physically realizable non-negative fluences are obtained by an iterative process of adjusting an input dose map in light of the actual dose produced by the calculated fluences.
U.S. Pat. No. 5,291,404, discloses a radiotherapy treatment planning system for calculating the radiation dose to be absorbed by an object to be irradiated, prior to radiotherapy. The distribution of the contribution output unit provides the distribution of contribution showing the contribution rates of scattered beam or electron beam to an observational point in each point of the object to be irradiated. An arithmetic unit calculates the absorbed dose of the observational point due to the scattered beam or the absorbed dose due to the electron beam, by summing up each contribution rate multiplied with the electron density at the corresponding point.
U.S. Pat. No. 5,216,255, discloses a system for applying radiation treatment undercomputercontrol. The system has a radiation source which generates a variable intensity radiation beam, and a collimator. The collimator has a plurality of movable plates disposed in the path of the radiation beam and is oriented in a direction perpendicular to the beam axis. The apparatus is capable of actuating the plates independently during the radiation treatment, in response to a first control signal. The beam changes in width when the plates are so actuated. The collimator is rotated in response to a second control signal. The intensity of the radiation beam may be varied as a function of the plate position. A total radiation dosage is applied during two intervals. The first interval precedes the collimator rotation, and the second interval follows the rotation.
U.S. Pat. No. 5,027,818, discloses a technique for computing the doses at various points within the patient""s body. In particular, the doses are computed at a relatively high density of points within a fine dose grid and at a relatively low density of points within a coarse dose grid. In that fashion, the user can quickly obtain necessary information about the radiation dose distribution before implementation of a proposed treatment plan. A technique of locating the intersection between the radiation beam and the contour or other surface of the patient is also provided. The method appears well suited for use with a particular structure which allows one to utilize relatively narrow beam widths as a result of great mechanical accuracy.
The foregoing patents are hereby incorporated herein by reference.
Prior art dual energy linear accelerators on the market today typically include a low energy beam (usually six megavolts or xe2x80x9cMVxe2x80x9d) and a higher energy beam (usually a fifteen MV). Obtaining beam energies between six and fifteen MV has been very difficult to achieve, often requiring the purchase of an intermediate energy linear accelerator. The purchase price of a single energy linear accelerator is currently a little more than half the price of a dual energy linear accelerator. Thus, the cost differential between single energy linear accelerators and dual energy linear accelerators is quite significant. This creates a practical economic barrier for most health care facilities. There is a need for a method of providing a large range of energy levels from available linear accelerators.
The present invention provides a method for producing a broad range of therapeutic radiation energy levels with a source of more than one value of radiation energy, e.g., a dual energy linear accelerator. In one embodiment of the inventive method, a dual energy linear accelerator is operated to produce photons at a first energy that are directed at a treatment site for a first absorbed dose. The linear accelerator is then operated to produce photons at a second energy directed at the treatment site for a second absorbed dose. The appropriate selection of the first and second absorbed doses yields an effective absorbed dose at the treatment site equivalent to the dose produced by photons having an energy that is intermediate of the first and second energies. Alternatively, the appropriate selection of the first and second doses may be presented in the form of dose characteristics (dose distributions) at the treatment site equivalent to the dose characteristics (dose distributions) produced by photons having an energy that is intermediate of the first and second energies. Thus, a four MV and fifteen MV dual energy linear accelerator may be operated according to the method to yield a continuous range of energies, e.g., 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 MV photons or particles. In another embodiment, an absorbed dose at the treatment site may be produced corresponding to the application of non-integer photon energies.
In another embodiment, a dual energy linear accelerator is operated to produce simultaneously photons at a first energy and photons at a second energy where the ratio of first energy photons to second energy photons is set according to a predetermined proportion. This composite stream of photons is then directed at a treatment site. The appropriate selection of the ratio of first energy photons to second energy photons imparts a photon dose at the treatment site equivalent to the dose produced by photons having an energy that is intermediate of the first and second energies.
In yet another embodiment, a dual energy linear accelerator is operated so as to produce photons at a first energy, which are directed at a treatment site to impart a fraction of a first absorbed dose. This operation is repeated until a whole first absorbed dose has been applied to the treatment site. The dual energy linear accelerator is then operated so as to produce photons at a second energy, which are directed at the treatment site to impart a fraction of a second absorbed dose. This operation is repeated until a whole second absorbed dose has been applied to the treatment site. The first and second whole absorbed doses are selected so as to impart an effective absorbed dose at the treatment site equivalent to an absorbed dose produced by photons having an energy that is intermediate of the first and second energies.
In a further embodiment, a dual energy linear accelerator is operated to produce photons at a first energy that are directed at a treatment site from a first direction to impart a first absorbed dose. The linear accelerator is then operated to produce photons at a second energy directed at the treatment site from a second direction to impart a second absorbed dose. The appropriate selection of the first and second absorbed doses and directions yields an effective absorbed dose at the treatment site equivalent to the dose produced by photons having an energy that is intermediate of the first and second energies.